Conventionally, systems of every kind which employ a current differential relay device for the purpose of protecting a power transmission line of an electric power system have been proposed. FIG. 4 is a configuration diagram of an example of such a conventional power transmission line protection system and, in particular, of a transmission path duplex system in which transmission paths are duplexed.
In the power transmission line protection system shown in FIG. 4, two current differential relay devices 1 (1a and 1b) are installed at the respective terminals 3 (slave end 3a and master end 3b) of a two-terminal power transmission line 2. The two current differential relay devices 1 capture quantity data of electricity of the electric power systems from the current transformers 4 (4a and 4b) of each terminal 3, are connected to each other via two transmission paths 5 and 6, and perform current differential operations by transmitting and receiving the quantity data of electricity of the respective terminals to each other.
Alphabetic characters “a” and “b” which are appended to the final letters of reference numerals which represent the respective constituent elements in FIG. 4 are used in order to differentiate between the slave end 3a and master end 3b of the two-terminal power transmission line 2 as well as between the two current differential relay devices 1a and 1b which are installed at the respective terminals 3a and 3b. In this case, the configurations of the two current differential relay devices 1a and 1b are mostly the same with some exceptions and, apart from the appended alphabetic characters “a” and “b”, the constituent elements indicated by the same reference numerals have the same configuration. Hence, in the following description, the appended characters “a” and “b” appear only in cases where such clear differentiation is required from the perspective of simplification and are suitably omitted in cases where such differentiation is unnecessary.
The current differential relay devices 1 each has an analog/digital conversion means (A/D conversion means) 11 which first captures quantity data of electricity of the electric power systems from the current transformers 4 in the form of analog inputs and performs analog/digital conversion on the quantity data of electricity, a reception changeover means 12 for performing reception changeover between the two transmission paths 5 and 6, and a transmission means 13 for communicating with the current differential relay device 1 at the other end of the power transmission line via the reception changeover means 12. The transmission means 13 is provided with a disturbance detection unit 131 which detects transmission defects in the transmission paths 5 and 6.
In a conventional transmission path duplex system of the kind shown in FIG. 4, the transmission means 13 receives data from the reception side transmission path which corresponds with the reception changeover by the reception changeover means 12 and transmits data in parallel to the two transmission paths 5 and 6. Further, in cases where a transmission defect in the current reception side transmission path is detected by the disturbance detection unit 131, the transmission means 13 transfers a transmission defect detection signal which indicates the transmission defect to the reception changeover means 12 to make it perform reception changeover. The details of the transmission path duplex system will be described subsequently.
The current differential relay devices 1 each further has a current differential operation means 14 which receives quantity data of electricity of its own end of the power transmission line from the analog/digital conversion means 11, receives quantity data of electricity of the other end of the power transmission line which are received by means of the transmission means 13, and performs a current differential operation by using these quantity data of electricity; a sequence operation means 15 which uses the current differential operation result to perform logic sequence processing and generate a trip signal; and an output means 16 which outputs the trip signal thus generated to a circuit breaker 7 at its own end of the power transmission line. As the current differential operation means 14, specifically “87 relays” or the like are used.
In addition, the current differential relay device 1a of the slave end 3a has a timing difference measurement means 17a for measuring the sampling pulse signal timing difference, a correction means 18a for calculating a correction amount for achieving synchronization with the sampling pulse signal of the master end 3b on the basis of the measured timing difference, and a sampling pulse generation means 19a for generating a sampling pulse signal which is sampling-synchronized with the master end 3b and transferring the sampling pulse signal to analog/digital conversion means 11a. In contrast, the current differential relay device 1b of the master end 3b has only timing difference measurement means 17b and sampling pulse generation means 19b. 
In the power transmission line protection system in FIG. 4, although a sampling pulse signal is transferred from the sampling pulse generation means 19 to the analog/digital conversion means 11 in order to capture quantity data of electricity of the line, the sampling pulse signal of the current differential relay device 1a of the slave end 3a is synchronized with the sampling pulse signal of the master end 3b by the correction means 18a. The sampling pulse signal timing difference is calculated by the timing difference measurement means 17a and 17b in accordance with the data from the transmission path connected by the reception changeover means 12.
Here, in the current differential relay device 1a of the slave end 3a, a sampling pulse signal correction amount for achieving synchronization with the sampling pulse signal of the master end 3b is calculated by the correction means 18a on the basis of the timing difference between the slave end 3a and master end 3b, and a sampling pulse signal which is sampling-synchronized with the master end 3b is generated by the sampling pulse generation means 19a. 
As specific sampling synchronization methods, the technologies which appear in Patent Document 1 and Patent Document 2, for example, have been proposed.
[Principles of Sampling Synchronization]
The principles of the sampling synchronization of the current differential relay devices of the master end and the slave end will be described hereinbelow by using FIG. 5(a) and FIG. 5(b). From the standpoint of simplification, the “master end” and “slave end” in the drawings and in the following description are abbreviations for “the current differential relay device of the master end” and the “current differential relay device of the slave end” and “device” is likewise an abbreviation for “current differential relay device”.
FIG. 5(a) shows a state where sampling is carried out by both a first terminal (the master end) and another terminal (the slave end) using a fixed sampling cycle T, and a sampling pulse signal timing difference of ΔT is arising between the master end and the slave end.
In FIG. 5(a), firstly, at the slave end, a synchronization signal (a sampling synchronization flag) is transmitted to the master end together with quantity data of electricity (F0). At the master end, a reception timing difference Tm from the sampling timing of its own device until the reception timing of data containing the sampling synchronization flag is measured, and the sampling synchronization flag and the measured reception timing difference Tm are sent back to the slave end together with the quantity data of electricity (F1).
Subsequently, similarly to the master end, the slave end measures the reception timing difference Ts from the sampling timing of its own device until the reception timing of the data containing the sampling synchronization flag and reads the reception timing difference Tm measured at the master end.
Here, supposing that an upstream transmission delay time up until the transmission data of the slave end arrive at the master end and a downstream transmission delay time up until the transmission data of the master end arrive at the slave end are both Td and equal, the transmission delay time Td can be expressed by the following Expressions (1) and (2) respectively by using the reception timing difference Tm or the reception timing difference Ts measured at the master end or the slave end, the sampling pulse signal timing difference ΔT, and the sampling cycle T.Td=Tm+ΔT+iT  (1)Td=Ts−ΔT+jT  (2)where i and j are both integers.In FIG. 5(a) and FIG. 5(b), i=1 and j=2.
Here, when the difference between Expressions (1) and (2) is taken and an arrangement as to ΔT is made, the following Expression (3) is obtained.2ΔT=Ts−Tm+(j−i)T  (3)
In addition, since the sampling pulse signal timing difference ΔT is, due to its very nature, always smaller than the sampling cycle T, the term (j−1)T in Expression (3) (that is, the multiple of T) can be eliminated. Hence, the following Expression (4) for calculating the sampling pulse signal timing difference ΔT is obtained from Expression (3).ΔT=(Ts−Tm)/2  (4)
Therefore, at the slave end, the sampling timing between the two mutually independent devices of the master and slave ends can be equalized by calculating Expression (4) and shifting the sampling timing of the slave end so that the sampling pulse signal timing difference ΔT is substantially zero. At the master end, the determined sampling pulse signal timing difference is used only in the judgment of whether sampling synchronization is to be implemented and correction of the sampling pulse is not performed.
FIG. 5(b) shows a state where the sampling timings of the master end and the slave end match each other. In this case, the slave end measures a sampling frequency T0 from the point where the sampling synchronization flag is sent until the sampling synchronization flag is sent back by the master end and calculates the transmission delay time Td by using the following expression from the sampling frequency T0, the timing difference Ts measured beforehand, and the sampling cycle T.Td=T0/2−T+Ts  (5)
At the slave end, since it is judged and become clear by using Expression (5) to what extent the data sent back from the master end are delayed with respect to the sampling timing of its own end, it is possible to perform an operation in which data sampled at the same time at the slave end and the master end are used.
[Transmission Path Duplex System]
A transmission path duplex system of the kind shown in FIG. 4 will be described in detail hereinbelow.
Generally, the operating ratio (the temporal probability of the protection function functioning normally) of a current differential relay device which protects a power transmission line by using a transmission path depends largely on the quality of the transmission path. That is, when a transmission defect arises in a transmission path, the current differential relay device basically prevents a malfunction by locking the current differential operation means. The protection function therefore stops and the operating ratio is reduced depending on the function stoppage.
In contrast, a transmission path duplex system is a system which has been proposed in order to improve the operating ratio of a current differential relay device by shortening the function stoppage time of the current differential relay device, even in cases where a transmission path of a quality which is not ideal is utilized. The transmission path duplex system connects two current differential relay devices provided at two terminals to each other via a duplexed transmission path.
In the conventional transmission path duplex system shown in FIG. 4, the transmission path is duplexed through two transmission paths 5 and 6, where one transmission path is applied for regular use and the other transmission path is applied for standby. Here, a case where transmission path 5 is applied for regular use and transmission path 6 is applied for standby will be described by way of an example. Transmission signals are outputted in parallel to the two transmission paths 5 and 6.
In the current differential relay devices 1a and 1b shown in FIG. 4, data are normally received from the other end of the power transmission line via the regular use transmission path 5 by connecting transmission means 13a and 13b to the regular use transmission path 5 via reception changeover means 12a and 12b of the current differential relay devices 1a and 1b. However, when a transmission defect arises in the regular use transmission path 5, current differential operation means 14a and 14b are locked.
A confirmation time of the order of ten seconds, for example, is provided in order to prevent excessive transmission path reception changeover when a transmission defect arises in the regular use transmission path 5. Disturbances in the transmission paths are detected by disturbance detection units 131a and 131b of the transmission means 13a and 13b within the confirmation time and it is judged whether transmission path reception changeover is required due to a disturbance. In cases where reception changeover is required, the transmission defect detection signal is transferred to the reception changeover means 12a and 12b, and reception changeover from the regular use transmission path 5 to the standby transmission path 6 is performed automatically by the reception changeover means 12a and 12b. As a result of the reception changeover, the lock on the current differential operation means 14a and 14b is released and the current differential operation can be continued.
When such a reception changeover system is employed, even in the case of a current differential relay device provided for a single transmission path, there is no need to change the transmission control and current differential operation mechanisms in any way and the current differential relay device can be easily made compatible with a transmission path duplex system simply by providing transmission path reception changeover means, which represents a major advantage in implementation.
As shown in the Expression (4) for calculating the sampling pulse signal timing difference ΔT, the sampling pulse signal timing difference ΔT does not depend on the transmission delay time Td of the transmission path. Hence, even though the transmission delay time of the one transmission path 5 and the other transmission path 6 are Td1 and Td2 respectively and are different from each other, the same sampling pulse signal timing difference ΔT is calculated as indicated by the following Expressions (6) to (13) from the respective reception timings Ts1, Tm1 and Ts2 and Tm2.
Therefore, during transmission path reception changeover, even when the transmission delay times of the two transmission paths 5 and 6 differ, there is no need to recapture the sampling signal (for example, in cases where the transmission path reception changeover is carried out when the sampling pulse signal timing difference ΔT=0, if the slippage while the sampling synchronization is interrupted due to a transmission defect is removed, the sampling pulse signal timing difference ΔT which is newly calculated in the transmission after changeover is also zero).
During selection of transmission path 5:Td1=Tm1+ΔT+iT  (6)Td1=Ts1−ΔT+jT  (7)ΔT=(Ts1−Tm1)/2  (8)Td1=T0/2−T+Ts1  (9)During selection of transmission path 6:Td2=Tm2+ΔT+iT  (10)Td2=Ts2−ΔT+jT  (11)ΔT=(Ts2−Tm2)/2  (12)Td2=T0/2−T+Ts2  (13)[Patent Documents]Patent Document 1:Japanese Patent Publication No. H1-890Patent Document 2:Japanese Patent Publication No. H1-24014
In a power transmission line protection system of a conventional transmission path duplex system of the kind mentioned hereinabove, it is necessary to provide the transmission path reception changeover with a confirmation time in order to prevent excessive transmission path reception changeover as mentioned hereinabove. However, when a transmission defect arises in the regular use transmission path, the power transmission line protection function of the current differential relay device stops during the confirmation time up until the reception changeover into the standby transmission path is completed.
That is, in the case of a conventional power transmission line protection system of the kind shown in FIG. 4, when a transmission defect arises in the regular use transmission path, as mentioned earlier, after a disturbance in the transmission path is detected by the disturbance detection units 131a and 131b of the transmission means 13a and 13b within the confirmation time and it is judged that reception changeover of the transmission path is required, transmission path reception changeover by the reception changeover means 12a and 12b is necessary and for the confirmation time in this case, for example, time of the order of ten seconds, for example, is required. Hence, during the confirmation time up until the transmission path reception changeover is completed, the power transmission line protection function of the current differential relay device stops and the operating ratio of the current differential relay device drops.
Furthermore, in cases where an intermittent transmission defect arises, transmission path reception changeover is not carried out and repeat of the stoppage and operation of the power transmission line protection function in the current differential relay device occur sometimes. In this case, the operating ratio of the current differential relay device also drops.